During liquid-liquid extraction, an organic reagent solution is mixed, in an extraction cell (mixer-settler) or in a column, into an aqueous solution, which contains a substance to be purified and concentrated as soluble, metal usually in ion form or as a complex together with several impurities. The valuable metal or substance to be refined reacts selectively with an organic extraction chemical, whereby it is separated from the aqueous solution into the extraction chemical as a pure form. The valuable metal or substance can then be separated from the organic solution back into the aqueous solution (stripping) with the inverted chemical reaction to extraction, from which aqueous solution it can be recovered again as a product e.g. by precipitation or reduction into metal.
The extraction process is thus mixing liquids physically insoluble in each other into droplets or a dispersion in the mixing section of the extraction equipment, and after chemical mass transfer occurs the droplets in the dispersion are made to coalesce i.e. to recombine back into the original layers of liquid in the settling section or settler. Intensive mixing or a significant change in the surface chemistry conditions of the process may result in very small droplets, which require a lot of time to disengage to its own liquid phase. These droplets do not necessarily have time to disengage in the actual settling section of the extraction step, but move further on in the process with the other phase. The inclusion of the original feed solution (aqueous solution) in the organic solution as it enters later process stages may weaken the purity of the final product and demand extra purification measures. Likewise the organic extractant may end up lost with the treated aqueous solution. In both cases the cost efficiency of the process is lessened.
In particular, a tank has been used for the removal of aqueous entrainment from a organic solution after the extraction cells, in which the entrained water droplets fall towards the bottom of the tank due to the force of gravity and the purified surface layer can be routed to the next process stage, which tank is then called an after-settler. The tank can also function simultaneously as a surge tank, which is needed to even out changes in the volume of organic solution occurring in various parts of the process. In this case the surface level of the solution in the tank varies.
The actual purification method of the organic solution, scrubbing, occurs using mixer-settler cells, in which basically the chemically bound impurities are removed by treating the organic solution with suitable aqueous solutions. In this case therefore a dispersion of extraction solution and aqueous solution is formed in order to achieve a large liquid-liquid surface area, as in an extraction cell. Besides chemical scrubbing, water droplets are also removed or the impurities contained in them are diluted. A mixer-settler cell built for scrubbing purposes generally consists of a pump, a mixer and a settling tank with its retaining fences, and is usually the size of an extraction cell. Changes in organic solution volume cannot be balanced with a scrubbing cell, so a separate surge tank as mentioned above is needed, which has the required volume capacity.